It is no use telling the hawks that bombing a country in which al-Qaida was not operating was unlikely to rid the world of al-Qaida. It is no use arguing that had the billions spent on the war with Iraq been used instead for intelligence and security, atrocities such as last week's attacks in Istanbul may have been prevented. As soon as one argument for the invasion and occupation of Iraq collapses, they switch to another. Over the past month, almost all the warriors - Bush, Blair and the belligerents in both the conservative and the liberal press - have fallen back on the last line of defence, the argument we know as "the moral case for war".

Challenged in the Commons by Scottish Nationalist MP Pete Wishart last Wednesday over those devilishly uncooperative weapons of mass destruction, for example, Tony Blair dodged the question. "What everyone should realise is that if people like the honourable gentleman had had their way, Saddam Hussein, his sons and his henchmen would still be terrorising people in Iraq. I find it quite extraordinary that he thinks that that would be a preferable state of affairs."

I do believe that there was a moral case for deposing Saddam - who was one of the world's most revolting tyrants - by violent means. I also believe that there was a moral case for not doing so, and that this case was the stronger. That Saddam is no longer president of Iraq is, without question, a good thing. But against this we must weigh the killing or mutilation of thousands of people; the possibility of civil war in Iraq; the anger and resentment the invasion has generated throughout the Muslim world and the creation, as a result, of a more hospitable environment in which terrorists can operate; the reassertion of imperial power; and the vitiation of international law. It seems to me that these costs outweigh the undoubted benefit.

But the key point, overlooked by all those who have made the moral case for war, is this: that a moral case is not the same as a moral reason. Whatever the argument for toppling Saddam on humanitarian grounds may have been, this is not why Bush and Blair went to war.

A superpower does not have moral imperatives. It has strategic imperatives. Its purpose is not to sustain the lives of other people, but to sustain itself. Concern for the rights and feelings of others is an impediment to the pursuit of its objectives. It can make the moral case, but that doesn't mean that it is motivated by the moral case.

Writing in the Observer recently, David Aaronovitch argued in favour of US intervention, while suggesting that it could be improved by means of some policy changes. "Sure, I want them to change. I want more consistency. I want Bush to stop tolerating the nastystans of Central Asia, to tell Ariel where to get off, to treat allies with more respect, to dump the hubristic neo-cons..." So say we all. But the White House is not a branch of Amnesty International. When it suits its purposes to append a moral justification to its actions, it will do so. When it is better served by supporting dictatorships like Uzbekistan's, expansionist governments like Ariel Sharon's and organisations which torture and mutilate and murder, like the Colombian army and (through it) the paramilitary AUC, it will do so.

It armed and funded Saddam when it needed to; it knocked him down when it needed to. In neither case did it act because it cared about the people of his country. It acted because it cared about its own interests. The US, like all superpowers, does have a consistent approach to international affairs. But it is not morally consistent; it is strategically consistent.

It is hard to see why we should expect anything else. All empires work according to the rules of practical advantage, rather than those of kindness and moral decency. In Arthur Koestler's Darkness at Noon, Rubashov, the fallen hero of the revolution, condemns himself for "having followed sentimental impulses, and in so doing to have been led into contradiction with historical necessity. I have lent my ear to the laments of the sacrificed, and thus became deaf to the arguments which proved the necessity to sacrifice them." "Sympathy, conscience, disgust, despair, repentance and atonement", his interrogator reminds him, "are for us repellent debauchery".

Koestler, of course, was describing a different superpower, but these considerations have always held true. During the cold war, the two empires supported whichever indigenous leaders advanced their interests. They helped them to seize and retain power by massacring their own people, then flung them into conflicts in which millions were killed. One of the reasons why the US triumphed was that it possessed the resources to pursue that strategy with more consistency than the Soviet Union could. Today the necessity for mass murder has diminished. But those who imagine that the strategic calculus has somehow been overturned are deceiving themselves.

There were plenty of hard-headed reasons for the United States to go to war with Iraq. As Paul Wolfowitz, the deputy defence secretary, has admitted, the occupation of that country permits the US to retain its presence in the Middle East while removing "almost all of our forces from Saudi Arabia". The presence of "crusader forces on the holy land" was, he revealed, becoming ever less sustainable. (Their removal, of course, was Osama bin Laden's first demand: whoever said that terrorism does not work?) Retaining troops in the Middle East permits the US to continue to exercise control over its oil supplies, and thus to hold China, its new economic and political rival, to ransom. The bombing of Iraq was used by Bush to show that his war on terror had not lost momentum. And power, as anyone who possesses it appreciates, is something you use or lose. Unless you flex your muscles, they wither away.

We can't say which of these motives was dominant, but we can say that they are realistic reasons for war. The same cannot be said of a concern for the human rights of foreigners. This is merely the cover under which one has to act in a nominal democracy.

But in debating the war, those of us who opposed it find ourselves drawn into this fairytale. We are obliged to argue about the relative moral merits of leaving Saddam in place or deposing him, while we know, though we are seldom brave enough to say it, that the moral issue is a distraction. The genius of the hawks has been to oblige us to accept a fiction as the reference point for debate.

Of course, it is possible for empires to do the right thing for the wrong reasons, and upon this possibility the hawks may hang their last best hopes of justification. But the wrong reasons, consistently applied, lead at the global level to the wrong results. Let us argue about the moral case for war by all means; but let us do so in the knowledge that it had nothing to do with the invasion of Iraq.